The present invention relates to processing seismic data, and is particularly concerned with avoiding normal moveout stretch during the stages of normal moveout correction and common-midpoint stacking.
Referring to FIG. 1 there is illustrated a known setup 10 for gathering seismic exploration data. A seismic sound source 12 is generated at or just below the earth""s surface 14, or in the case of marine seismic, just below the water surface. For each such source generation, or xe2x80x9cshotxe2x80x9d, the sound travels into the earth 16, reflects off of changes in geology (called xe2x80x9ceventsxe2x80x9d or xe2x80x9creflectorsxe2x80x9d) 18, and travels back to the surface 20, where it is simultaneously recorded at a plurality of receivers 22. A single such recording at a receiver 22, is called a xe2x80x9ctracexe2x80x9d and is in the form of a regularly sampled time series measuring the particle velocity (for land data) or change in acoustic pressure (for marine data). A single shot is typically recorded at hundreds or thousands of receivers simultaneously. Many such shots are taken for a single seismic data set, sometimes resulting in hundreds of millions of seismic traces.
Seismic data can be used to interpret, or infer, geology, and thus is useful for the location, identification, and exploitation of petroleum and minerals. Before it can be used for this purpose, however, seismic data must undergo a series of statistical processes, a task generally referred to as xe2x80x9cseismic processingxe2x80x9d.
There are a number of effects that can confound the ability to interpret seismic data. One such effect is noise, defined generally as any unwanted recorded energy. The origins of noise can be both natural and man-made, much of the noise being caused by the shot itself.
A second confounding effect is multiples, which is energy that has reflected more than once in its propagation from shot to receiver. A typical multiple propagation path is illustrated in FIG. 2. Seismic energy travels from the shot 12 down 24 to a reflector 26, back up 28 to the surface 14, back down 32 to a second reflector 18, and then back up 34 a receiver 36 where it is recorded.
The desired energy for seismic exploration is singly reflected, that is energy that has reflected from only one geological reflector. Events that have reflected only once before being recorded, are referred to as xe2x80x9cprimariesxe2x80x9d.
One step in seismic processing is normal-move-out (NMO) correction. Traces with different xe2x80x9coffsetsxe2x80x9d (the horizontal distance between shot and receiver) have the same reflection (or event) appearing at different times. NMO correction is a time-variant shifting of sample values so that each trace""s reflections are aligned to occur at the same time, that is, as if the trace had zero offset. The NMO correction allows for stacking of traces with different offsets.
The principal parameter involved in applying NMO correction is a stacking velocity, which is a single value, varying in both time and space, controlling the amount of time shift. Choosing these stacking velocities is a routine part of seismic processing that occurs for virtually every data set.
For a given zero-offset time within a single common midpoint (CMP) gather, a traditional NMO correction shift follows a hyperbolic curve as a function of offset. In recent years more complicated formulas have been introduced that compensate for near-surface effects (Link, et al, 1992), anisotropy (change in velocity with propagation direction), and vertical velocity gradient. To correct for these last two effects, the seismic processor must pick a NMO correction parameter xcex7 (Alkhalifah, 1998) in addition to a stacking velocity.
Another step in seismic processing is front-end muting, which is the setting to zero of sample values near the beginning of the trace. The purpose of front-end muting is to remove noise and other unwanted effects from the front of the trace.
Another step in seismic processing is common-midpoint (CMP) stacking, where traces, having roughly the same midpoints between their shot and receiver positions, are collected into groups. At each recorded time sample, the non-muted values for every trace in the group are averaged together, producing a single xe2x80x9cstackedxe2x80x9d trace for each group. One benefit of CMP stacking is noise reduction due to the averaging of many values into one value. A second benefit is multiple reduction, resulting from the fact that while NMO correction lines up primary reflections, it does not line up multiple reflections, so that multiples tend to attenuate during averaging.
The multiple-reducing property of stacking depends critically on there being a broad range of offsets within the non-muted sample values. Another-benefit of stacking is reducing the amount of data, typically by a factor between 10 and 100, so that the data can be displayed in a manner that is convenient and easy to interpret, specifically as a xe2x80x9cstacked CMP sectionxe2x80x9d. Yet another benefit of stacking is reduction of the computation time required for later processes such as migration and noise reduction.
The typical processing steps of seismic processing are illustrated in FIG. 3. We begin with a CMP gather of traces 42. A NMO correction is applied 44, and then front-end muting 46. Finally the traces are stacked into a single stacked trace 48.
NMO correction causes distortion of seismic events in time (Dunk and Levin, 1973), the principal effect being stretch, which is the conversion of high frequencies into low frequencies by expanding the time base. It is a well known principle in seismic processing that to be as interpretable as possible, seismic events should be as broad band in frequency as the noise allows (Berkhout, 1984). Thus NMO stretch can mean a loss in the ability to infer geology. NMO stretch is particularly severe at early times, large offsets, and fast vertical changes in velocity. Referring to FIG. 4, there is illustrated an artificial CMP gather 50. After NMO correction 52 the gather has stretch 54 at far offsets and early times. After stacking the gather 56, there is distortion of early events 58 as compared to later events 60.
NMO stretch is caused by the implicit assumption that seismic events occur instantaneously. However, this is not the case. A seismic event typically has an effective length between 20 to 60 ms in duration. As a result, during standard NMO correction, a different time shift is applied to the beginning of an event than to the end.
A well known way of avoiding NMO stretch is to apply a front-end mute that zeroes all trace samples suffering from too much NMO stretch. FIG. 5 illustrates a is known seismic processing sequence. The same artificial NMO-corrected gather 52 as in FIG. 4 is shown. A front-end mute is applied 64. The resulting stack 66 shows much less distortion at early times 68 as compared to later times 70.
However, there are drawbacks to this approach. First, the CMP gather has less redundancy (or xe2x80x9cfoldxe2x80x9d) at early times, resulting in decreased noise reduction in stacking. Second, the CMP gather has less far-offset information at early times, resulting in decreased multiple reduction in stacking. Third, changes in event character with offset contains valuable interpretive information referred to as amplitude-versus-offset, or AVO effects (Castagna and Backus,1993). Front-end muting results in the loss of some AVO information.
Other solutions have been suggested for mitigating NMO stretch. Rupert and Chun (1975) introduced block move sum NMO, where traces are subdivided into overlapping blocks of samples. Each block has constant-shift NMO applied, and the blocks summed with weights to form the NMO-corrected gather. A related approach was described by Shatilo and Aminzadeh (2000), where the normal moveout function is kept constant in the vicinity of discrete events. Byun and Nelan (1997) apply time-varying filters to NMO-corrected traces to reverse the loss of high frequencies.
Hicks (2001) describes a method for removing NMO stretch during stacking based on a Parabolic Radon Transform, as well as a method that removes stretch from an NMO-corrected CMP gather based on a Fourier-Radon Transform. There are a few drawbacks to these methods; first, the processor must pick an appropriate path through the transform results, or rely on an automatic picker whose robustness in the face of noise is questionable; second, they give poor results for overlapping events with different velocities; third, the methods may give poor results for subtle events which are not picked, and whose presence the processor may not even be aware of.
An object of the present invention is to provide an improvement for normal moveout correcting and stacking of seismic traces.
In accordance with an aspect of the present invention there is provided a method of correcting for normal moveout and stacking of seismic traces comprising the steps of: inputting a common midpoint gather; muting a front-end portion of each trace; defining a series of overlapping time intervals; solving for a time interval fit to gather data; positioning time intervals at their zero-offset positions; and summing time intervals at each time sample to provide a stretch-free stacked trace.
In accordance with an aspect of the present invention there is provided apparatus for correcting for normal moveout and for stacking of seismic traces comprising: means for inputting a common midpoint gather; means for muting a front-end portion of each trace; means for defining a series of overlapping time intervals; means for solving for a time interval fit to gather data; means for positioning time intervals at their zero-offset positions; and means for summing time intervals at each time sample to provide a stretch-free stacked trace.